Microwave transmission line termination

ABSTRACT

A power dissipating termination for microwave transmission lines is disclosed. Nontapered, high-loss dielectric material replaces the relatively lossless dielectric material immediately preceding the end of a two-conductor line. The high-loss material has a dimensional configuration which results in a substantially nonreflective termination.

United States Patent Graham 5] Apr. 41, 1972 [5 1 MICRGWAVE TRANSMISSION LINE 3,254,316 5/1966 McHenry ..333/22 TERMINATION 3,509,495 4/1970 Morton ..333/22 3,541,474 11/1970 Holton ..333/22 [72] Inventor: Bobby Eagle Graham, Greensboro, NC. [73] Assignee: Bell Telephone Laboratories, Incorporated, OTHER PUBLICATIONS Murray Hill, Berkeley g King, Minmo and Wing, Transmission Lines Antennas and [22] Filed: June 29, 1970 Waveguides McGraw-Hill New York 1945 T145741; Title Page and pp. 6- 9,13, 66- 67 [21] Appl. No.: 50,560

Primary Examiner-Herman Karl Saalbach 52 us. c1. ..333/22 11, 333/81, 333/84 M 51 1111. C1 ..r101 l/26,H01p 3/08 Guemher and wllllam Keefauve' [58] Field ofSearch ..333/22, 81, 81 A, 84, 84 M,

333/9 [57] ABSTRACT A power dissipating termination for microwave transmission [56] References cued lines is disclosed. Nontapered, high-loss dielectric material UNITED STATES PATENTS replaces the relatively lossless dielectric material immediately preceding the end of a two-conductor line. The high-loss 3,505,619 4/ 1970 material has a dimensional configuration which results in a 2,471,732 5/1949 substantially nonreflective termination. 2,828,469 3/1958 2,961,621 11/1960 5Claims,3Drawing Figures 14 15 2 I6 m m 1 j flm '9 I a a xxxavmwxxxxx; 2 v -,/|8 17 l 13 II *1 l2 PATENTEDAFR 4 I972 3,654, 573

FIG. 2

//v l EN 70/? 9. E. GRA HA M ATTORNEY GOVERNMENT CONTRACT The invention herein claimed was made in the course of or under a contract with the Department of the Army.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to power dissipating terminations for microwave transmission lines.

2. Description of the Prior Art In the prior art, power dissipating terminations for microwave transmission lines have been formed using resistive or high-loss dielectric materials. To be effective, these terminations must have adequate power dissipating abilities and should introduce little, if any, reflections. As appreciated by those skilled in the art, this latter characteristic requires any discontinuity between a line and a termination to be small.

The above-identified low reflection requirements present a problem when using high-loss dielectric material in the manner taught by the prior art. In particular, the prior art teaches that the high-loss dielectric material must be tapered over a distance of a wavelength or greater so as to keep reflections to a minimum. Providing a taper, however, adds to the overall length of the termination. This results in an overly long termination at frequencies between one and six GHz.

SUMMARY OF THE INVENTION An object of the present invention is to terminate a transmission line so as to dissipate, in a relatively short termination, power at one or more GI-lz while producing little, if any, reflections.

In accordance with the invention, nontapered, high-loss (i.e., energy-absorbing) dielectric material replaces the relatively lossless dielectric material immediately preceding the end of a two-conductor transmission line. The high-loss dielectric material replaces the low-loss dielectric material for an effective electrical length l of the line so that an abrupt change in dielectric materials occurs and, furthermore, so that h l in ll Z: In Z" when the end of the line is short circuited and in COIIT 70 when the end of the line is open circuited, where Z the characteristic impedance of the line containing the relatively lossless dielectric;

2,, the characteristic impedance of the line containing the energy absorbing dielectric; and y the propagation constant of the line with the energy absorbing dielectric. As a practical matter, the energy absorbing material is chosen so that the values for 2,, and result in a value for l which has a negligible, if any, imaginary part.

Embodiments of the invention may comprise individual units which are electrically connected to the end of a line or may form an integral part of the line. In the disclosed embodiments the termination is an integral part of a stripline transmission line.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 and 3 are fragmentary views of two embodiments of the invention; and

FIG. 2 is a cross-sectional view of these embodiments.

DESCRIPTION OF THE DISCLOSED EMBODIMENTS The invention is disclosed using stripline configurations. Although it readily lends itself to this type of construction, it is equally applicable to other forms of two-conductor microwave transmission lines.

FIG. 1 shows an embodiment of the invention as an integral portion of an open end stripline transmission line, while FIG. 2 is a cross-sectional view taken at line 2-2 of FIG. 1. These views are not to scale but instead show some elements, as will become apparent, in an exaggerated form for purposes of clarity. Furthermore, these views should be considered together when reading the following description.

The embodiment of FIG. 1 includes a metallic housing 10 having a cavity therein. Several layers of materials reside in this cavity. The lower-most layer comprises a rectangular section 11 of low-loss dielectric material with a rectangular section 12 of high-loss dielectric material abutting it. On top of sections 11 and 12 is a strip 13 of Mylar material on which is bonded a narrower, constant-width strip 14 of copper. The next layer comprises a rectangular section 15 of low-loss dielectric material with a rectangular section 16 of high-loss dielectric material abutting it. Sections l1, l2, l5 and 16 all have substantially the same cross-sectional dimensions. These materials protrude slightly above the upper edge of the cavity so that they are held tightly in place by a three member cover comprising a copper sheet 17, a sheet 18 of compressible R.F. shielding material and a metallic cover plate 19, all of which are bolted in that order, to housing 10. A coupler 20 is affixed to the end of housing 10 opposite to that containing sections 12 and 16.

As recognized by those skilled in the art, copper strip 14 forms the center conductor while housing 10 and copper sheet 17 form the ground planes of a stripline transmission line. Microwave energy is coupled to the line by way of coupler 20 while, in accordance with the invention, that energy is dissipated in sections 12 and 16. The heat generated by this dissipation is rapidly conducted away by the metallic structure surrounding sections 12 and 16.

Sections 11 and 15 and sections 12 and 16 meet in a plane perpendicular to copper strip 14. This abrupt physical discontinuity provides a substantial space saving when compared with the tapered discontinuity taught by the prior art. In accordance with the invention, this abrupt physical continuity is permissible because a low electrical reflection discontinuity is achieved by selecting the penetration of copper strip 14 into sections 12 and 16 so that the reactive component presented by the termination is essentially zero over a reasonably broad frequency band about the operating frequency. In particular, the lengths of these elements are selected so that the termination has an effective r.f. length 1 defined by the following expression:

where Z, the characteristic impedance of the line containing the relatively lossless dielectric;

2 the characteristic impedance of the line containing the energy absorbing dielectric; and

'y the propagation constant of the line with the energy absorbing dielectric.

Any difference between effective length l and the actual length of copper strip 14 between sections 12 and 16 occurs as a result of energy fringing at the end of copper strip 14. When the material of sections 12 and 16 is of a nature which results in little fringing, these lengths are substantially equal to one another. The difference between actual and effective lengths produced as a result of fringing is well known and is easily ac commodated by those skilled in the art.

Fifty-ohm embodiments as shown in FIG. 1 and for use at 3 01-12 have been built and tested. The following materials were used: sections 11 and 15 were each one-eighth of an inch thick and were formed from a material having a relative dielectric constant of 2.53; sections 12 and 16 were each one-eighth of an inch thick and were formed from a material known as Eccosorb MP 500 F l 17 (having a relative dielectric constant of 21.8) and manufactured by Emerson and Cummings, Inc of Canton. Massachusetts; sheet 18 was a material identified by the trademark Tech-felt and manufactured by Technit, Inc. of Cranford, New Jersey; strip 13 was of Mylar material having a thickness of 0.005 inch, strip 14 was a copper strip bonded to strip 13 and having a thickness of 0.0028 inch; and housing 10 and plate 19 were made from aluminum. Because very little energy fringing occurred within the material used for sections 12 and 16, the effective length l and the actual physical length of copper strip 14 within these sections differed by only 2 or 3 mils. The actual physical length used was 0.185 inch. Measurements on the resulting configuration showed a standing wave ratio (SWR) of less than 1.22 over a frequency band equal to 16 percent of the operating frequency of an SWR of less than 1 .12 over an 8 percent frequency band.

Another embodiment of the invention is shown in H6. 3. This embodiment is very similar to that of FIG. 1 and consequently the numerals used in FIG. 1 have been used again to identify the various elements of this embodiment. This embodiment differs from that of FIG. 1 in that copper strip 14 is in electrical contact with housing 10, thus providing a shorted line. The expression for the effective length l for this embodiment is the same as the previous expression with the exception that tanh" replaces "coth."

As previously mentioned, the disclosed embodiments use center conductors having constant widths and dielectric materials having substantially the same cross-sectional dimensions. To be able to construct terminations in this manner is, of course, highly desirable. The widths of the center conductors and/or the cross-sectional dimensions of the dielectric materials in the terminations are not, however, restricted to those in the low-loss line. in particular, they may differ so as to achieve a value of Z which will result in the desired electrical length I.

What is claimed is:

l. A stripline transmission line for use as a power dissipating termination, said line comprising a pair of parallel metallic ground planes,

a center conductor positioned parallel to said ground planes and substantially midway therebetween,

a low-loss dielectric material between said ground planes and said center conductor for at least a portion of the length of said center conductor, and

a high-loss dielectric material between said ground planes and said center conductor for the remainder of the length of said center conductor so that the juncture between said dielectric materials lies in a plane perpendicular to said center conductor and so that said remainder of the length of said center conductor has an effective electrical length [in accordance with the expression tanh iq" when the end of said line is short circuited and when the end of said line is open circuited, where Z the characteristic impedance of said portion of said line containing said low-loss dielectric material;

Z, the characteristic impedance of said remainder of said line containing said high-loss dielectric material; and

y the propagation constant of the line with said high-loss dielectric material.

2. An energy absorbing termination for one end of a transmission line having a relatively lossless dielectric between two parallel conductors, said termination comprising energy absorbing dielectric material replacing said relatively lossless dielectric material along a length of said line preceding said end of said line so that the juncture between said dielectric material lies in a plane perpendicular to said conductors and, furthermore, so that the portion of said line including said energy absorbing dielectric has an effective electrical length I in accordance with the expression when said end of said line is short circuited and coth- I 1 7 when said end of said line is open circuited, where Z the characteristic impedance of said line containing said relatively lossless dielectric material;

Z, the characteristic impedance of said portion of said line containing said energy absorbing dielectric material; and

y the propagation constant of said line with said energy absorbing dielectric material.

3. Means for absorbing the energy at the end of a microwave transmission line having inner and outer parallel conductors, said means comprising high-loss dielectric material in the portion of said line immediately preceding said end with the innermost surface of said material lying in a plane perpendicular to said center conductor and said center conductor encased by said material for an effective electrical length Idefined by the expression tanh- I II Z in l coth 0 when said end of said line is open circuited, where Z, the characteristic impedance of the portion of said line not containing said high-loss dielectric material;

Z the characteristic impedance of said portion of said line containing said high-loss dielectric material; and 7 the propagation constant of said line with said high-loss dielectric material. 4. A microwave transmission line termination comprising a microwave transmission line segment having inner and outer parallel conductors, coupler means connected to one end of said line segment,

and high-loss dielectric material in the portion of said line segment preceding its other end with the innermost surface of said material lying in a plane perpendicular to said center conductor and said center conductor encased by said material for an effective electrical length I defined by the expression termination, said line comprising when said other end of said line segment is short circuited the juncture between said dielectric materials lies in a and plane perpendicular to said center conductor, and

ground planes parallel to said center conductor and im- Zm mediately adjacent to said dielectric material, 60 h 7 5 said high-loss dielectric material surrounding said center l=" conductor for a length to efiect an electrical length I in 7 accordance with the expression when said other end of said line segment is open circuited, [anh '1 where Z the characteristic impedance of the portion of 10 I: said line segment not containing said high-loss dielectric 7 material;

Z the characteristic impedance of said portion of said when the end Ofsaidline is Short circuited and line segment containing said high-loss dielectric material;

and Goth L -y the propagation constant of said line with said high-loss I:

dielectric material. y

5. A stripline transmission line for use as a power dissipating when the end of said line is open circuited,

where Z, the characteristic impedance of said portion of said line containing said low-loss dielectric material;

2., the characteristic impedance of said remainder of said a center conductor. a low-loss dielectric material having a rectangular cross section and surrounding said center conductor for at least a portion of the length of Sal-d center conductor line containing said high-loss dielectric material, and

a high-loss dielectric material having a rectangular cross Tithe P Q constant ofsaid line with said high'loss section and surrounding said center conductor for the delecmc matena'l' remainder of the length of said center conductor so that 

1. A stripline transmission line for use as a power dissipating termination, said line comprising a pair of parallel metallic ground planes, a center conductor positioned parallel to said ground planes and substantially midway therebetween, a low-loss dielectric material between said ground planes and said center conductor for at least a portion of the length of said center conductor, and a high-loss dielectric material between said ground planes and said center conductor for the remainder of the length of said center conductor so that the juncture between said dielectric materials lies in a plane perpendicular to said center conductor and so that said remainder of the length of said center conductor has an effective electrical length l in accordance with the expression when the end of said line is short circuited and when the end of said line is open circuited, where Zin the characteristic impedance of said portion of said line containing said low-loss dielectric material; Zo the characteristic impedance of said remainder of said line containing said high-loss dielectric material; and gamma the propagation constant of the line with said highloss dielectric material.
 2. An energy absorbing termination for one end of a transmission line having a relatively lossless dielectric between two parallel conductors, said termination comprising energy absorbing dielectric material replacing said relatively lossless dielectric material along a length of said line preceding said end of said line so that the juncture between said dielectric material lies in a plane perpendicular to said conductors and, furthermore, so that the portion of said line including said energy absorbing dielectric has an effective electrical length l in accordance with the expression when said end of said line is short circuited and when said end of said line is open circuited, where Zin the characteristic impedance of said line containing said relatively lossless dielectric material; Zo the characteristic impedance of said portion of said line containing said energy absorbing dielectric material; and gamma the propagation constant of said line with said energy absorbing dielectric material.
 3. Means for absorbing the energy at the end of a microwave transmission line having inner and outer parallel conductors, said means comprising high-loss dielectric material in the portion of said line immediately preceding said end with the innermost surface of said material lying in a plane perpendicular to said center conductor and said center conductor encased by said material for an effective electrical length l defined by the expression when said end of said line is short circuited and when said end of said line is open circuited, where Zin the characteristic impedance of the portion of said line not containing said high-loss dielectric material; Zo the characteristic impedance of said portion of said line containing said high-loss dielectric material; and gamma the propagation constant of said line with said high-loss dielectric material.
 4. A microwave transmission line termination comprising a microwave transmission line segment having inner and outer parallel conductors, coupler means connected to one end of said line segment, and high-loss dielectric material in the portion of said line segment preceding its other end with the innermost surface of said material lying in a plane perpendicular to said center conductor and said center conductor encased by said material for an effective electrical length l defined by the expression when said other end of said line segment is short circuited and when said other end of said line segment is open circuited, where Zin the characteristic impedance of the portion of said line segment not containing said high-loss dielectric material; Zo the characteristic impedance of said portion of said line segment containing said high-loss dielectric material; and gamma the propagation constant of said line with said high-loss dielectric material.
 5. A stripline transmission line for use as a power dissipating termination, said line comprising a center conductor, a low-loss dielectric material having a rectangular cross section and surrounding said center conductor for at least a portion of the length of said center conductor, a high-loss dielectric material having a rectangular cross section and surrounding said center conductor for the remainder of the length of said center conductor so that the juncture between said dielectric materials lies in a plane perpendicular to said center conductor, and ground planes parallel to said center conductor and immediately adjacent to said dielectric material, said high-loss dielectric material surrounding said center condUctor for a length to effect an electrical length l in accordance with the expression when the end of said line is short circuited and when the end of said line is open circuited, where Zin the characteristic impedance of said portion of said line containing said low-loss dielectric material; Zo the characteristic impedance of said remainder of said line containing said high-loss dielectric material; and gamma the propagation constant of said line with said high-loss dielectric material. 